A method for evaluating an effective component content (C) of a reducing agent for engine exhaust gas processing arranged in a container (205) in which a heat transfer provision arrangement (240) is provided, including steps of: determining (s410; s420) a prevailing volume (V) and temperature (T1) of the reducing agent in the container (205); determining (s430) a prevailing temperature (T2) of the heat transfer provision arrangement (240); determining (s440) a prevailing temperature (T3) of a medium surrounding the container (205); for a predetermined time period, determining (s450) a mean temperature change rate (Tprim) for the reducing agent; and determining (s460) the effective component content (C) of the reducing agent on the basis of the above determined parameters.